Due to an increasing awareness of global energy needs and environmental pollution in recent years, much interest has been devoted to the development and use of thermoelectric (TE) materials for automotive and other applications. TE devices are capable of transforming heat directly into electrical energy and also acting as solid state coolers. Through their energy-generating capability, TE devices are capable of enhancing the ability of internal combustion engines to convert fuel into useful power. The cooling capability of TE devices can contribute to a resolution of the greenhouse concerns associated with refrigerant use, as well as enable new design concepts for heating and air conditioning and improve the reliability of batteries. TE-based waste heat recovery is also applicable to modes of transportation such as diesel-electric locomotives, locomotive diesel engines, automotive diesel engines, diesel-electric hybrid buses, fuel cells, etc.
The energy conversion efficiency and cooling coefficient of performance (COP) of a TE device are determined by the dimensionless figure of merit, ZT, defined as ZT=S2T/ρKtotal=S2T/ρ(KL+Ke) where S, T, ρ, Ktotal, KL, and Ke are the Seebeck coefficient, absolute temperature, electrical resistivity, total thermal conductivity, lattice thermal conductivity and electronic thermal conductivity, respectively. The larger the ZT values, the higher the efficiency or the Coefficient of Performance (COP). An effective thermoelectric material should possess a large Seebeck coefficient, a low electrical resistivity and a low total thermal conductivity.
Binary skutterudites are semiconductors with small band gaps of ˜100 meV, high carrier mobilities, and modest Seebeck coefficients. Binary skutterudite compounds crystallize in a body-centered-cubic structure with space group Im3 and have the form MX3, where M is Co, Rh or Ir and X is P, As or Sb. Despite their excellent electronic properties, binary skutterudites have thermal conductivities that are excessively high to compete with state-of-the-art thermoelectric materials. It was found that filled skutterudites have much lower thermal conductivities. Therefore, filled skutterudites are increasingly popular as a thermoelectric material due to their lower thermal conductivities.
Filled skutterudites can be formed by inserting rare earth guest atoms interstitially into large voids in the crystal structure of binary skutterudites. The chemical composition for filled skutterudites can be expressed as GyM4X12, where G represents a guest atom, typically a rare earth atom, and y is its filling fraction. Compared to binary skutterudites, the lattice thermal conductivities of the rare earth filled skutterudites are significantly reduced over a wide temperature range. This property of filled skutterudites is due to the scattering of heat-carrying low-frequency phonons by the heavy rare earth atoms, which rattle inside the interstitial voids in the skutterudite crystal structure.
In recent years, both n- and p-type rare earth filled skutterudites have been reported to have superior thermoelectric figure of merit (ZT) values in excess of 1 for temperatures above ˜500 degrees C. For rare earth filled skutterudites, the best p-type materials are La—Fe—Co—Sb and Ce—Fe—Co—Sb skutterudites. The best p-type materials are Yb—Co—Sb and Ba—Ni—Co—Sb. FIG. 1 shows ZT values of recently-discovered filled skutterudites as compared to those of state-of-the-art thermoelectric materials.
The relatively high cost of high-purity starting materials for rare earth filled skutterudites contributes to the overall cost of the fabricated thermoelectric devices. Therefore, filled skutterudites are needed which utilize low-cost starting materials to decrease the overall cost of thermoelectric devices.